Software Modems » History » Revision 4
Revision 3 (laforge, 06/29/2022 08:03 AM) → Revision 4/11 (laforge, 06/29/2022 08:07 AM)
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h1. Software Modems
(sometimes also called _WinModems_)
h2. What is a Software Modem
Traditional analog modems are based around hardware modulators/demodulators, typically some kind of DSP or VLSI chipset including some kind of internal processor / software. They are basically an embedded system with two external interfaces:
* the analog phone line interface
* a RS-232 serial line towards a terminal or computer
As desktop computers and laptops became more powerful around the turn of the century, various manufacturers started to implement _software modems_. Those modems only contained the analog line interface and an ADC/DAC. All the actual signal processing (equalizer, modulation, demodulation, coding/decoding, etc.) happened in software running on the main processor of the desktop/laptop computer (typically an x86 CPU).
The related proprietary software for the main CPU was typically specific to a given vendor/chipset, and was offered in most cases only for Windows type operating systems, hence also the name _WinModem_. There were some exceptions where binary-only software was also offered for Linux or other OSs.
h2. Models/Types of Software Modems
h3. FIXME
h3. slmodemd
h3. MC97 / AC97 Modems
At some point, the AC97 standard for mainboard-based _sound card_ functionality was extended so that the audio software stack could not only drive the analog audio codec for multimedia purposes, but also drive a specific "telephone line interface codec IC" with associated analog line interface. This meant that the modem would basically show up as a second audio device on the PC.
The actual "telephone line interface codec IC" together with the analog interface circuitry was often not put on the mainboard of the PC or Laptop itself, but implemented via an add-on card. See [[AMR_Modem_Riser]] for more information.
h2. Open Source Software for Software Modems
There were several different projects that implemented software modem functionality. The lack of a standard interface (software API or hardware interface) towards the analog line circuitry
h3. spandsp
"spandsp":https://github.com/freeswitch/spandsp (originally released on https://www.soft-switch.org/) is a library of DSP functions for telephony created by Steve Underwood.
Its primary use case was to send/receive Fax over analog phone lines in the software VoIP/PBX systems (iaxmodem, Asterisk, CallWeaver and FreeSwitch)
h3. linmodem
[[linmodem:]] is a softmodem implemented by "Fabrice Bellard":https://bellard.org/ originally released at https://bellard.org/linmodem/ - some Osmocom mebers have done some small improvements to make it build on mordern Linux distributions and have plans to further extend it, see the "issue tracker":https://osmocom.org/projects/linmodem/issues and "git repository":https://gitea.osmocom.org/retronetworking/linmodem
h3. ArduinoModem
This is not really a _contemporary_ software modem, but a retrocomputing project by @supersat
"ArduinoModem":https://github.com/supersat/ArduinoModem is a project implementing Bell 103 (300 bps) on an arduino microcontroller. It allegedly is loosely based on https://web.archive.org/web/20120110020118/http://www.ti.com/lit/an/slaa037/slaa037.pdf and https://web.archive.org/web/20130512113608/http://www.ti.com/lit/an/slaa204/slaa204.pdf
